\section{The A5/3 algorithm}

After it was discovered that the $A5/1$ and $A5/2$ encryption algorithms in fact
was vulnerable to attacks the $A5/3$ was developed. Where the two previous
versions specifications was kept secret, the $A5/3$ was published. 

The $A5/3$ takes the same input as $A5/1$ and $A5/2$ \cite{a53spec}, namely the
$K_c$ and $COUNT$, and outputs two 114 bit keys. The algorithm is based on the KASUMI block cipher,
which uses an eight round Feistel network. 

Since KASUMI is a block cipher, it uses output feedback (OFB) mode to
generate the key-stream. The way this mode works can be seen in \rf{OFB}

\begin{figure}[htp]\label{OFB}
\centering
\includegraphics[scale=0.389]{OFB.png}
\caption{Overview of the OFB mode setup}
\end{figure}

The stream is here generated by the block cipher that takes an IV and runs the
output of each encryptions through to the next block, to create the key-stream.
